No Marley is not a dog - quite the opposite so no pun intended. Marley the actress was cast for a particular reason - for a few that is. One of them being her looks and of course another one her ability to act. I am aware that some may feel that she may not be the best in that job category.
That being said, she is more than good enough for the role she has to play here (again no pun intended). Having a Dennis Hopper at her side, who has fun with his role (quite obviously and understandably so) helps too of course.
I reckon most will be able to deduct and guess where the movie is heading. Towards the end it even tells us clearly what happened and what went on - and yet, the inevitable has to happen ... it is what it is. A neat little thriller.